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Stage Lighting and Stadium Seating Assemblies

A top manufacturer of truss systems consulted SPIROL on an existing design of their truss system. The design was comprised of one tubular structural member and one solid male member that slid inside the tubular extrusion and was cross-pinned with two high carbon steel Slotted Pins to hold the coupler to the frame. During installation, the Slotted Pin was difficult to install and would often damage the expensive, yet relatively soft, aluminum frame. Once the hole was deformed, the manufacturer noted instances where the pin would begin to walk out of the hole over time in permanent installations. This was also noted in mobile systems if subjected to extreme loading. In some instances they had also noticed that the Slotted Pin had stress cracks located 180 degrees from the slot. In extreme weather conditions, the high carbon steel pin was also prone to corrosion. With a lack of options, the manufacturer had investigated implementation of an expensive machined Solid Pin with a secondary retention feature of a Cotter Pin. SPIROL was asked to evaluate the application and ultimately recommend the most robust and cost effective solution for this incredibly demanding application.

Slotted Pin tended to work harden directly opposite the gap resulting in stress failures and pin walk-out.

Flexibility of the Coiled Pin during and after installation eliminates the risk of catastrophic failure due to hole damage or fatigue.

SPIROL’s Engineering Department determined that 420 stainless steel Coiled Pins were a high performance alternative to the Slotted Pin. Since the Coiled Pin does not need to be aligned in the shear plane like a Slotted Pin, this eliminated the potential of failure due to pin orientation in the hole. Since Coiled Pins remain flexible in the application after installation, they are able to absorb dynamic loading rather than transferring it into the host, which can cause deformation. The reduction in deformation increased the life of the expensive structure, and also reduced the potential of the holes opening up to the extent that the pin would eventually fall out. Since a 420 stainless steel Coiled Pin is heat treated, its shear value exceeds that of a machined low carbon steel Solid Pin. Complexity of the assembly was kept to one component rather than two, as proposed with the Solid Pin and Cotter Pin solution; thus saving time during set-up and tear down as well as the task of managing multiple components.

By utilizing 420 series stainless steel, the manufacturer would also achieve the necessary corrosion resistance, and would ultimately yield more reliable performance over the life of the structure. An added bonus was that no adjustment to the hole size would be required to transition from the Slotted Pin to the Coiled Pin. This was a benefit for both the manufacturer and their customers that already had truss systems in service throughout the country. Implementation of the Coiled Pin ultimately met all of the manufacturer’s performance and commercial requirements and significantly increased the Integrity of the assembly.

The following benefits were realized by conversion to a Coiled Pin in this application:
• Flexibility after installation absorbs loading rather than transmitting it to the host.
• Coiled Pins reduce the complexity of the assembly vs. the machined Solid Pin/Cotter Pin solution.
• Coiled Pins do not need to be aligned in the shear plane like Slotted Pins. This eliminates potential installation errors and greatly reduces the potential for failures.
• Coiled Pins distribute the load evenly throughout the pin vs. Slotted Pins that focus stress 180 degrees from the slot.
• Coiled Pins have the widest hole tolerance of any press fit fastener.

SPIROL Products in this Application Story

Coiled Spring Pins

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